Question in Electro- Optics. 391 



The extent of the jump, taken at every 5th turn of the plate, 

 was now ^ the fringe-width; and at every 10th or 15th 

 turn, it was clearly 4. I find in my notes that this large 

 jump of the fringes impressed me here, again and again, as a 

 thing peculiarly beautiful. 



Fourth Set, the same again as the second. The only ques- 

 tion in this case was, whether it might still be possible, by the 

 most careful work and under the best conditions attainable, 

 to detect a very small jump of the fringes at the instant of 

 discharge. Many observations were taken at high potential, 

 some at the highest, but without a trace of effect of that 

 kind. 



Fifth Set, the same again as the first. The results of first 

 and third sets were recovered regularly. Sparks were then 

 taken, sometimes at every turn of the plate, sometimes oftener. 

 At every spark there was a very small downward jump of the 

 fringes, so small sometimes as to be barely caught, but quite 

 regular and beautifully distinct. 



Remarks. — The jump of the fringes was chosen as the 

 principal object of observation, because it was never quite 

 concealed, nor even much obscured, by the mechanical dis- 

 turbance of the liquid ; but I should add that the contrary 

 motion, the gradual ascent of the fringes during the process 

 of charging, was generally evident enough in the experiments, 

 though not often undisturbed or quite regular in its course. 



The best observations were got when the fringes happened 

 to continue at rest through a sensible interval of time, including 

 the instant of discharge. The contrast between the two cases 

 was then very remarkable, especially at high potential : in the 

 one case, the beautifully clear jump so often mentioned ; in 

 the other case, no trace of a jump in either direction, generally 

 not even a perceptible shiver of the fringes at the instant of 

 strongest discharge. Instances of this kind occurred not very 

 rarely in the experiments; and there could be no contrast more 

 striking than that between the phenomena in the two cases. 



From what I know of the apparatus and its performance, 

 I am sure that no regular and abrupt retardation or accelera- 

 tion amounting to as much as the hundredth part of an average 

 wave-length could have escaped observation in the experiments. 

 It will be remembered also that the jump of the fringes at 

 high potential extended to four fifths of the fringe-width. 

 With reference, therefore, to the dielectric CS 2 , and the two 

 principal vibrations parallel and perpendicular to the line of 

 force, it appears that the regular effect of the electric strain 

 upon one of the vibrations is a positive retardation, while 

 upon the other vibration there is very probably no effect 



2 D2 



